Compressor Fridge

[bobalobs]

Our current van has an underfloor gas tank but we are finding increasingly difficult to find autogas stations so are looking to reduce our gas consumption.

If we change our van the direct replacement could be fitted with a 85 L compressor fridge with two 110 ah gel leisure batteries and a 115 W solar panel.

We try to be off hook up except for the 5 winter months. How long will the batteries last without van movement or hookup?

We are not convinced that lithium is the way forward .

[Derek Uzzell]

It ought to be possible to make a reasonably accurate estimate of the average daily electrical demand of a particular 85 litre compressor fridge. This prediction would be based on the data provided by the fridge’s manufacturer.

 

However your question "How long will the batteries last without van movement or hookup?” isn’t really answerable as the 2 x 110Ah batteries would undoubtedly also be powering other 12V ‘leisure’ items (lights, water-pump, heater fans in cool weather, etc.)

 

This 2019 forum thread might be of interest

 

https://forums.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/3-way-v-compression-fridge/51680/

[mikefitz]

Based on my experience of using and installing 12v compressor fridges, the 12/24 volt units designed for the leisure market have similar performance for a given size. All use the same 3 phase compressor with electronic control module. The control electronics has a variable speed option that some fridge manufactures use for a ' night' or 'reduced power' mode.

 

The running current of the compressor at 12v is 2.5 to 5 amps depending on the speed selected by the manufacture, thus allowing the same unit to be used in different sized fridges. All have active cooling of the condenser radiator with a small 12v fan. Unlike 3 way gas fridges there is no need for external vents in the van side. Installing with adequate air flow to and from the heat exchanger is relatively easy. The electrical side of the installation requires 6mm2 cable from the leisure battery fused at 15 amps.

 

Electrical power consumption will vary with fridge size, ambient temperature, fridge contents and use, typically a mid sized unit will take 4 amps at 12 V and cycle with a 30% on time, 30 Ah per day.

Following the rule of a 50% maximum discharge for lead acid batteries, a 100 Ah fully charged battery will last almost two days with no charging, disregarding other 12v loads. A lithium battery of the same 100 Ah capacity would easily last 3 days.

 

Over the summer months in the UK a roof mounted 100 watt solar panel with good solar conditions could yield, with a quality MPPT controller, around 35 Ah per day. This will be lower with a low cost PWM controller often fitted as part of the electrical system. Shade or heavy cloud will reduce solar output to a low value.

 

Experience and feed back from users suggests that with 200 Ah of LA batteries and 200 watts of solar panels is sufficient for almost continual 'off grid' in summer with good weather..

 

Batteries play an important part in a compressor fridge installation. Unfortunately most LA batteries in a motor home will have a reduced capacity after a year or so use due to incorrect charging and accelerated aging. This inadequacy may/will compromise the operation of a compressor fridge.

 

There is also a misunderstanding of how long a LA battery need to be charged, a 50% discharged battery will take at least 5 hours to fully charge, regardless of the 'power' of the charger. Up to around 80% capacity the charge rate is fast, above this the battery takes less and less current , with the current at almost zero at full charge. Voltage alone is not an indication of full charge.

 

Off grid use will need an efficient and fast charging method for engine charging. This means a Battery to Battery charger ideally needs to be installed, ( always needed for the latest Euro6 vehicles), rated at up to 30 or 60 amps. Most LA batteries have a recommended charge current limit of 10% to 15% of capacity, say 15 A per 100 Ah.

 

A lithium battery should be considered when up grading the electrical system, due to the fast charging ability and the fact that you can use almost all the capacity.

 

I can confidently say that if you experience a compressor fridge with a suitable electrical and battery system you will never again consider a 3 way gas fridge.

I am also confident that a change to a lithium battery system will be a 'positive' experience.

 

Mike

 

[bobalobs]

Sorry for the delay as been away. Very helpful advice. I should have mentioned that the heating for the van water and interior is by a Webasto Dualtop so the expense of lithium may be justified.

You mention the efficiency of LA batteries reducing over their life. Lithium suppliers talk of a lifespan of ten years plus. At what stage in their life would they begin to deteriorate or do they just " fall off a cliff"?

[mikefitz]

bobalobs - 2021-07-20 10:43 AM

 

You mention the efficiency of LA batteries reducing over their life. Lithium suppliers talk of a lifespan of ten years plus. At what stage in their life would they begin to deteriorate or do they just " fall off a cliff"?

They age from the day they are made. The better cell manufacturer will usually specify 'cycles' full to empty at 2000 cycles under lab conditions at charge and discharge values at 0.5C, 50 amps per 100 Ah, The expected capacity after 2000 cycles is 80% of the 'new' capacity.

Ageing is increased by elevated temperature, charging/discharging in excess of 0.5C, holding the battery for long periods at full or empty.

From the test results and scientific papers I have seen it seems natural aging for a cell stored at room temperatures at 50% to 60% capacity, is low, less than 2% per year. Some lithium cells showed a loss of 10% capacity per year stored at 100% charge.

The feed back from my sailing contacts, many of which have been using lithium batteries for 10 years or more, supports these effects.

The batteries are usually 'retired' when they have lost 20% capacity, however for 'camper' use with modest charge and discharge needs, they would still continue to give service life. Although internal resistance rises with age they are still quite efficient, you get almost the same energy out that you put in.

I expect service life could easily reach 10 years with quality products, used and charged correctly, There are very few independent capacity tests available on the batteries on the UK market. I suspect some do not meet the rated capacity even when new.

I think the greatest factor that will reduce service life is the 'lead acid' mindset to keep batteries 'fully charged'. This, with the higher than idea voltages that chargers are set to, will have a negative effect on service life.

 

Mike

[bobalobs]

Many thanks again and more food for thought. Are you saying that lithium should never be left fully charged for any period of time? With lead acid I leave it fully charged until the next trip which in lockdown has been sometime. Is that wrong with lithium?

[colin]

When not in use Lithium are best part discharged, the problem is how do you ensure it goes away part discharged.

Unlike lead/acid, you would be better off running the battery down to say 80% using lights or fridge with solar switched off, then leaving it at that.

[mikefitz]

To add to the comment regarding lithium battery charge levels.

Some of the better offerings in the UK have BMS, battery protection modules, inside the casing, that have Bluetooth capability that allows battery capacity to displayed via a phone app. This or the addition of a battery monitor, a battery fuel gauge, like the Victron BMV712, allows battery 'housekeeping'.

 

Mike